Constant elongation rate tests (CERT) were conducted to evaluate the effect of heat treatment on intergranular stress corrosion cracking (IGSCC) susceptibility of alloy 600 (UNS N06600) in 140°C and 50% caustic solution at –900 mV vs saturated calomel electrode (SCE). Results showed:
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Heat treatment at low temperature for a long time (600°C for 260 h) led to a material that was not susceptible to caustic intergranular (IG) cracking. Increase in heat treatment temperature enhanced IG cracking susceptibility. Caustic IGSCC susceptibility was at maximum near the carbon solubility limit. However, when the heat treatment temperature was higher than the carbon solubility limit, a significant decrease in crack growth rate was observed.
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Grain boundaries acted as a preferential crack path when grain boundary carbon segregation was likely. Thermodynamic considerations suggested that severe caustic IGSCC susceptibility near the carbon solubility limit could be explained in terms of carbon segregation at the grain boundaries.
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IGSCC in caustic solution did not seem to be caused by chromium depletion.
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Although formation of semi-continuous IG carbides and IGSCC resistance seemed to exhibit a similar chronological response with heat treatment, it was unlikely that grain boundary IG carbides played a role in caustic IGSCC susceptibility.
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Carbon in solution enhanced the dissolution rate of alloy 600 significantly in 140°C, 50% sodium hydroxide (NaOH) solution at –900 mVSCE. The role of carbon in the dissolution behavior appeared to be to stabilize the formation of nickel hydroxide (Ni[OH]2) film, which has poor corrosion protection capability.
